J-   /^ 


^  ? 


Circular  Xo.  27.— Revised. 


United  States  Department  of  Agricult 


DIVISION    OF    BOTANY, 


CANADA   THISTLE. 

{Carduus  arvensis  (L.)  Robs. 


In 


the  history  of  weeds  in  America  none 


n^lM 



s 


/  '»n«f> 


bV^'  5i^Sii;oRY 


plained  of  than  the  Canada  thistle  (fig.  1). 
regarded  b}'  the  farmers  of  New 
P^ngland  as  the  greatest  pest 
of  their  fields.  When  the  sons 
of  these  farmers  moved  West 
the  Canada  thistle  went  with 
them.  It  grows  vigorously, 
sometimes  spreads  rapidlj^  and 
is  always  difficult  to  kill  by 
ordinary  cultivation.  It  forms 
dense  patches,  sometimes  to 
the  complete  exclusion  of 
other  plants,  and  its  abundant 
sharp  spines  make  it  disagree- 
able to  handle.  To  these  char- 
acters are  due  its  traditional 
reputation  in  tiie  Northeastern 
United  States  as  the  worst  of 
all  weeds.  With  little  d«ul)t, 
however,  it  causes  in  the  aggre- 
gate  less  real  injury  to  farm 
products  than  does  bull  thistle, 
ragweed,  or  pigeon  grass,  and 
its  distribution  if*  confined  to 
a  smaller  area  than  that  of  any 
of  these  weeds. 

XAMKS. 


A  century  ago  it  was 


1.  Canada  thistle,  showjiiy  horizontal  r«>()ts,  youii 
shoots,  anri  mature  plant  in  flower. 


The  phmt  received  the  name  p,, 
Canada  thistle  in  this  country 
soon  after  the  war  of  the  Revolution,  as  it  was  suprosed  to  have  been 
introduced  from  Canada.-  This  name  is  now  used  generally  in  both 
Canada  and  the  United  States.  In  England  the  plait  is  known  by  the 
nanies   corn   thistle,  green  thistle,   and  creeping  thistle.      In  Australi.i 


and  New  Zealand  it  is  often  called  California  thistle.  The  names  boar 
thistle,  cursed  thistle,  dog  thistle',  field  thistle,  and  waste  thistle  have 
also  been  applied  to  it. 

In  different  botanical  handbooks  published  during  the  past  twenty- 
five  j^ears  this  plant  has  ))een  described  under  three  different  technical 
names,  Carduus  arvettsis,  CirsiHm  arvense,  and  Cuicus  arveiisis. 
Recent  studies  upon  the  synonj-nn-  confirm  Card t( us  arveiisis  as  the 
correct  name. 

DESCRIPTION. 

Canada  thistle  is  usually  first  introduced  into  new  localities  by  the 
seed.     The  seed  germinates  and  a  rosette  of  leaves  lying  almost  fiat  on 

the  ground  is  first  formed. 
These  leaves  are  prickly  on  the 
margins,  somewhat  woolly  on 
the  under  surface,  but  green 
and  nearlj'  smooth  on  the 
upper.  The  following  year  a 
flowering  stalk  branching  at  the 
top  grows  up  to  a  height  from 
one  to  three  feet  (20  to  100cm.), 
rarel}'  higher.  The  stalk  is 
more  slender  than  that  of  most 
other  thistles  and  bears  very 
few  spines  (fig.  2,  n) .  The 
earlier  lower  leaves  are  four  to 
eight  inches  (10  to  20  cm.) 
long  and  about  one-fifth  as 
wide.  The  later  leaves  on  the 
upper  parts  of  the  stalks  and 
branches    are    smaller.       The 

Fi<i.  2.  Canada    thisth'.     iK'tail:    a.    main   stcni   and     ,  g  ,.       ,         ..  .„i:..,. 

leaf  :/..fl.,wor  head  :c.  seed  with  pappus,  natural  si..:     l^>wer     SUrfaCCS    ot     the    earlier 

rf.  seed,  eniartrixi.  with  pappus di'taiiu'd.  leavcs   are    at    lirst    somcwhat 

woolly,  l)ut/  the  upper  siufaces  of  all  the  leaves  are  bright  green  and 
smooth  or  slightly  hairy.  They  are  uneven  or  rufHed  and  the  margins 
are  irn^guhirly  toothed  and  very  spiny,  the  longer  spines  being  one-lifth 
to  thre(;-(;ighths  of  an  inch  (5  to  S  mm.)  long.  The  ilovvers  are  rose- 
I)urple,  rarely  white,  in  licads  one-half  to  seven-eighths  of  an  inch  (1.5 
to  2  cm.)  in  diameter,  clustered  at  the  ends  of  the  branches.  The 
green  bracts  surrounding  the  Mower  heads  (fig.  2,  h)  are  entirely  devoid 
of  hard,  stiff  prickles  or  spines.  I'sually  a  conii)aratively  small  number 
of  the  flowers  produce  seeds  (strictly,  akenes)  and  in  many  localities 
no  perfect  seeds  have  been  found.  The  abundant  white,  feathered 
I)api)us  or  down  (fig.  2,  c)  is  usually  fornKid,  however,  even  though  the 
seeds    are   not   developed.     The   seeds   are   smooth,    brown,    about  an 


I 


eighth  of  an  inch  (3  mm.)  in  length,  nearly  cjdindrical,  pointed  at  the 
lower  end  and  with  a  slight  projection  from  the  center  of  the  truncate 
circular  apex  (fig.  2,  d) . 

When  plants  grow  from  the  running  roots  in  the  fall  they  send  up 
shoots  with  rosettes  of  leaves  smaller  than  those  produced  by  the  seed- 
lings, but  when  they  grow  during  the  spring  and  summer  the  rosette 
stage  is  usually  omitted.  The  running  root  is  light  yellow  or  nearly 
white,  smooth,  cylindrical,  and  about  a  quarter  of  an  inch  (7  mm.)  in 
diameter  (fig.  1).  Numerous  rootlets  are  scattered  along  its  whole 
length,  but  there  are  no  nodes,  scales,  or  buds,  such  as  are  found  on 
the  underground  portions  of  the  shoots.  It  is,  therefore,  technically 
a  root,  and  not  a  rhizome  or  rootstock,  as  it  has  often  been  called. 
The  running  root  extends  horizontally  at  a  distance  of  three  inches 
to  three  feet  below  the  surface  of  the  ground,  the  lower  depths  being 
reached  usually  where  the  soil  is  deep  and  porous.  It  will  send  shoots 
to  the  surface  through  at  least  three  feet  of  hard-packed  soil.  This 
fact  has  been  repeatedly  noted  where  the  plants  on  vacant  lots  in 
Washington  have  been  covered  by  the  brick-like  soil  drawn  from  exca- 
vations and  thoroughly  packed  by  the  successive  cart-loads  hauled 
over  it. 

OTHER    THISTLES    MISTAKEN    FOR    CANADA    THISTLES. 

Several  other  thistles  are  often  mistaken  for  Canada  thistles.  As 
these  are  chiefly  annuals  or  biennials,  differing  very  much  from  the 
Canada  thistle  in  their  injurious  characters,  and  requiring  ver^-  different 
treatment  for  their  eradication,  it  is  important  that  they  be  distinguished. 

The  most  common  and  most  widely  distributed  of  these  is  the  bull 
thistle,  Carduus  lanceolatus  (fig.  3).  This  is  an  introduced  biennial 
species.  It  is  distributed  exclusively  by  the  seeds,  but  these  are  pro- 
duced in  such  great  numbers  that  the  plant  multiplies  very  rapidlj'. 
It  often  forms  patches  several  acres  in  extent  on  newly  cleared  land, 
but  in  old  fields  the  plants  are  usually  more  scattered.  It  seldom  per- 
sists in  any  great  quantity  and  is  readily  destroyed  by  cultivation.  It 
may  be  easily  distinguished  by  its  greater  size  and  much  larger  heads, 
with  spine-pointed  scales,  and  by  its  leaves  the  upper  surfaces  of  which 
are  always  rough,  like  a  cat's  tongue. 

In  the  South  Atlantic  States  bird's  nest  thistle,  Cardiui.s  spinosissi- 
mus,  is  often  mistaken  for  Canada  thistle.  This  is  a  native  perennial 
thistle,  which  blooms  early  in  spring  and  is  sometimes  troublesome  in 
early  crops  and  in  meadows.  It  is  not  regarded  as  a  serious  pest, 
however,  and  is  easily  eradicated  by  cultivation  or  by  increased  fertil- 
ization and  thick  seeding.  It  spreads  by  the  seeds  alone,  as  it  has  no 
running  roots.  The  bird's  nest  thistle  may  be  recognized  bj'  its  short, 
thick  stem,  only  10  to  20  inches  high,  bearing  at  the  top  two  to  five 
large  heads,  surrounded  by  very  spiny  pinnate  scales. 


mk 


On  the  Pacific  coast  milk  thistle,  Silybum  mariannm,  has  been 
mistaken  for  Canada  thistle.  Milk  thistle  is  an  introduced  annual 
plant  that  has  become  abundant  in  some  places  in  California  and  in 
waste  ground  about  eastern  cities.  It  maj^  be  recognized  by  its  coarse, 
thistle-like  habit,  large  leaves,  mottled  with  white,  and  milky  juice. 

The  curled  thistle, 
Cardutis  cri spu  s  , 
introduced  at  a  few- 
points  from  Vermont 
to  Pennsylvania,  and 
in  ballast  about  sea- 
ports, has  been  mis- 
taken for  the  Canada 
thistle,  and  in  this 
case  the  resemblance 
is  very  close.  It  has 
the  same  slender 
habit,  small  heads, 
and  light-colored  foli- 
age. The  best  char- 
acters for  distinguish- 
ing between  them  are 
the  prickly  winged 
stems,  the  spine- 
pointed  scales,  and 
the  plumeless  or  non- 
feathered  pappus  bris- 
tles of  the  curled  this- 
tle, as  compared  with 
the  feathered  pappus 
of  the  Canada  thistle. 
In  case  of  doubt  as 
t(^  any  plant  su])i)()sed 
to  be  Canada  thistle, 
tlic  )>est  method  is  to 
destroy  it  so  as  to  pre- 
vent seed  production  —  alter  mailing  to  the  Department  of  Agriculture 
for  identification  some  of  the  leaves  and  (lower  heads.  Specimens  of 
Canada  thisth',  or  of  plants  supposed  to  be  Canada  thistle,  are  especially 
(h'sired  from  localities  when'  its  presence  is  not  indicated  by  marks  on 
the  accompanying  map. 

IIISTOKV. 

Th(;  Canada  thistle  was  a  troublesonu!  weed   in  the  lields  of  southern 
Europe  as  early  at  least  as  the  beginning  of  tlie  sixteenth  century.     By 

the    middle  <»f   the   eighteenth    century    it    had    spread    throughout    the 


Via.  :{.  Hull  thistle,  n,  piece  of  main  stem,  with  leaf;  />,  llowei 
hea<l;  c,  seed,  with  pappus,  luituial  size;  r/,  see<l,  enlar>,'e<l.  with 
pappus  detached. 


greater  part  of  Europe,  and  now  it  is  found  in  western  Asia,  northern 
India,  Australia,  New  Zealand,  and  North  America.  In  all  regions 
w^here  it  has  become  naturalized  it  has  the  same  reputation  as  an 
aggressive  and  pernicious  weed. 

INTRODUCTION   INTO   AMERICA. 

Some  of  the  earlier  American  botanists  held  the  opinion  that  while 
the  Canada  thistle  had  doubtless  been  introduced  into  New  England 
from  Europe,  it  was  probably  indigenous  in  western  Canada.  It  now 
appears  very  improbable  that  it  is  indigenous  anywhere  on  the  Amer- 
ican Continent.  It  was  evidently  introduced  into  the  French*  settle- 
ments in  Canada  earlier  than  into  the  English  and  Dutch  colonies  of 
New  England  and  the  Middle  States.  It  is  reported  to  have  been 
found  about  the  residences  of  French  missionaries  in  Canada  early  in 
the  seventeeth  century.  There  is  a  tradition  that  it  was  purposely  intro- 
duced into  Canada  bj^  the  French  for  feeding  swine;  but  there  appears 
to  be  no  just  ground  for  this  tradition,  as  there  is  no  record  that  thistles 
were  ever  used  to  any  considerable  extent  as  food  for  swine  in  Europe. 

It  is  said  to  have  been  introduced  into  eastern  New  York  with  the 
hay  and  camp  equipage  of  Burgoyne's  army  in  1777.  It  probably 
reached  Vermont  at  the  same  time  or  previously,  as  it  was  recognized 
as  a  troublesome  weed  in  that  State  earlier  than  in  New  York.  The 
farmers  of  Vermont  had  become  so  greatly  alarmed  by  its  progress 
that  a  law  was  passed  by  the  legislature  in  1795  directing  its  destruc- 
tion on  all  lands  within  the  State.  In  1813  the  legislature  of  New 
York  passed  a  law  authorizing  certain  towns  to  pay  rewards  for  its 
destruction.  These  laws  were  well  enforced  at  first,  as  thej-  were 
passed  at  the  request  of  the  farmers  directl}'  interested.  But  the 
farmers  soon  learned  that  the  thistle  could  be  controlled,  and  then 
their  alarm  ceased  and  with  it  their  interest  in  the  complete  extermi- 
nation of  the  plant.  Since  then  successive  generations  have  attacked 
the  thistle  spasmodical!}',  soon  becoming  satisfied  of  their  abilitj"  to 
keep  it  in  check,  and  relaxing  their  efforts.  The  thistle  has  never 
relaxed,  however.  It  has  continued  to  grow  and  to  spread  each  year, 
although  it  has  been  exterminated  on  many  individual  farms.  As  early 
as  1850  it  had  spread  throughout  New  England,  Pennsylvania,  and 
eastern  Ohio,  and  had  gained  a  foothold  in  scattering  localities  in  many 
other  States.  During  the  war  its  range  was  extended  southward  all 
along  the  line  east  of  the  Missouri  River,  and  in  more  recent  years  it 
has  been  extending  westward. 

PKKSKXT    RANGE. 

The  Canada  thistle  is  now  found  from  Maine  to  Virginia  and  west- 
ward to  North  Dakota  and  Kansas,  and  on  the  Pacific  coast  from 
AVashington  to    northern    California.     Tlic    States    soutli    of  Virginia, 


6 

Kentucky,  Missouri,  and  Kansas  are  practically  free  from  it,  and  it  has 
become  established  in  only  a  few  localities  in  the  Rocky  Mountain 
region  and  Great  Basin.  It  is  abundant  and  troublesome  from  southern 
Maine  to  Maryland  and  westward  to  Indiana  and  Wisconsin,  and  in 
restricted  localities  in  some  of  the  States  bordering  this  area,  also  in 
the  Pacific  coast  States. 


NOT  LIKELY   TO    BE    TROUBLESOME    IX    THE    SOUTH. 

Very  few  reports  of  Canada  thistle  have  been  received  from  the 
Southern  States,  and  when  these  have  been  investigated  it  has  nearly 
always  proved  that  other  thistles  were  mistaken  for  Canada  thistle. 
The  true  Canada  thistle  was  introduced  several  years  ago  into  Apa- 


Kl 

^4-^A---^*^  ^iM 

/L   (^'J  '  'I    /  ^M6^"~(^r— 4J-__II] 

«» * 

ki 

)\L\      r    iP^~~~l"' 

^"^ 

^ — hCCa  / '"     /     '' — 

,-     r"    ^ 

s/ 

^                    ^             ^'     *«•    ^           li-                    W                    .^                     «r                     «•                     -*                     H-         J] 

Fig.  4.  Map  .showing  distribution  of  Canada  thistle  in  the  United  States  as  indicated  hy  reitorts 
received  at  the  Department  of  Ajfriculture.     +  Kecord  or  report.  °  specimens  scmi. 

lachicola,  Fla.,  l)ut  it  soon  died  out,  although  no  efforts  were  made 
to  destroy  it.  In  1898  it  was  intnxUured  on  ballast  ground  at  Mobile, 
Ala.,  l)ut  none  of  the  plants  bore  perfect  seeds  and  onh'  one  ]>lant 
appeared  in  1S94.  Canada  thistle  seeds  were  undoubtedly  carried 
to  many  parts  of  the  South  in  hay  during  the  war.  and  during  recent- 
years  the  increased  trafhc  between  the  Northern  and  Southern  States 
has  given  aljundant  opportunity  for  tb(?  introduction  of  thistle  seeds. 
These  facts  indicate  that  it  is  not  likely  ever  to  l^ecome  such  a  pest 
in  the  South  as  it  is  now  in  the  North.  It  may  be  expected,  liow- 
ever,  in  the  rich  valleys  in  the  Piedmont  regions,  where  the  clifuatic 
conditions  are  similar  to  those  of  the  mountain^  regions  of  rcnnsyl- 
vani;i. 


DANGER    OF   ITS   IXTRODUCTIOX    IX    XORTHERX    PRAIRIE    STATES   AND 
ROCKY   3I0UXTAIX    REGIOX. 

The  Canada  thistle  has  long  been  abundant  and  troublesome  in 
Manitoba  and  is  now  found  at  many  points  along  the  Canadian  Pacific 
Railway  from  Winnipeg  to  the  Pacific  coast.  In  most  of  the  localities 
where  it  has  been  introduced  in  Minnesota,  North  and  South  Dakota, 
Colorado,  and  Idaho,  it  thrives  and  shows  a  disposition  to  spread.  The 
climatic  and  agricultural  conditions  throughout  a  large  part  of  this  area 
preclude  the  employment  of  manj^  methods  for  combating  the  thistle 
which  are  in  common  use  in  the  East,  and  at  the  same  time  the  prac- 
tice of  irrigation  will  aid  in  disseminating  the  seeds.  Therefore,  if  the 
plant  is  allowed  to  become  widely  established  here,  it  promises  to  be 
even  more  troublesome  than  it  has  been  in  other  parts  of  the  country. 

3IETH0DS    OF   DISTRIBUTIOX. 

The  Canada  thistle  spreads  over  large  areas  or  travels  long  distances 
by  means  of  its  seeds.  It  spreads  into  patches  through  its  perennial 
running  roots.  Both  of  these  means  are  effective  in  their  vcay.  Perfect 
seeds  are  not  often  produced  until  after  the  plants  have  become  well 
established  and  have  spread  to  some  extent  by  the  running  roots.  In 
some  localities  in  Wisconsin  and  Iowa  close  observation  of  the  plants 
for  several  years  has  failed  to  discover  perfect  seeds.  The  plants  appear 
to  be  somewhat  erratic  in  this  respect,  however.  In  1894  very  few  were 
found  at  Washington  bearing  perfect  seeds,  while  in  1895  nearh^  all 
bore  perfect  seeds,  though  such  were  produced  by  less  than  half  of  the 
flowers  in  each  head.  In  1896  the  plants  were  again  seedless.  All 
three  seasons  were  alike  exceptionally  dry  for  this  region.  In  1899,  an 
abnormally  wet  season  in  Washington,  they  were  seedless,  and  no  seed- 
lings were  found  in  the  spring  of  1900.  There  were  no  apparent  fungus 
or  insect  enemies  to  account  for  failure  to  produce  seeds.  The  fact  that 
the  plants  are  imperfectly'  dioecious  does  not  fully  explain  these  differ- 
ences in  seed  production. 

DISSEMINATION    OF   SEEDS    BY    NATURAL    MEANS— WIND    AND   WATER. 

The  seeds  are  carried  from  farm  to  farm  by  the  wind,  and  along 
streams  they  are  carried  by  the  water.  The  seeds  are  mature  from 
midsummer  onward  through  the  season,  and,  as  thej'  are  easily  detached 
from  the  heads  by  slight  breezes,  most  of  them  are  disseminated  before 
the  hard  winds  of  late  autumn  and  winter  begin  to  blow.  The  feathered 
pappus  is  very  abundant,  and  the  seeds  are  comparatively  light,  so  that 
they  may  be  carried  a  mile — rarely  farther — in  the  windstorms  that 
often  precede  summer  thundershowers,  but  they  are  too  heavy  to  float 
upon  light  breezes  or  in  still  air.  Perfect  seeds  are  easily  detached  from 
the  pappus,  and  the  thistle  down  so  often  seen  floating  about  is  either 
that  from  which  the  seeds  have  already  fallen  or  that  from  flowers  which 


8 

did  not  bear  perfect  seeds.  The  distribution  caused  b\'  the  wind  is 
principally  from  waste  land  and  fence  rows  to  cultivated  fields  and  from 
field  to  field.  This  distribution  is  confined  to  short  distances,  and  is 
generally  in  the  direction  of  prevailing  winds. 

Every  rain  falling  on  a  hillside  thistle  patch  washes  the  seeds  down 
the  slope.  Seeds  from  a  patch  of  Canada  thistles  growing  in  a  moun- 
tain vallej'  are  disseminated  b^^  freshets  all  along  the  banks  of  streams 
below.  Transportation  by  water  becomes  a  special  danger  in  regions 
where  irrigation  is  practiced,  as  the  seeds  of  thistles  growing  on  the 
banks  of  rivers  or  irrigation  canals  will  float  down  the  streams  and 
ditches  and  be  deposited  in  the  fields  under  the  best  conditions  for 
propagation. 

DISSEMINATION    OV    SEEDS    BY    ARTIFICIAL    MEANS. 

The  dissemination  of  seeds  by  natural  means  accounts  in  part  for  the 
distribution  of  the  Canada  thistle  over  limited  areas,  but  were  it  not 
for  the  unwitting  or  careless  aid  of  man  its  progress  would  be  compara- 
tivel}'  slow.  The  seeds  were  first  brought  from  P]urope  to  America  by 
man.  They  have  been  transported  from  Europe  to  this  country  in 
impure  seed.  The  hay  or  straw  used  in  packing  the  cheaper  kinds  of 
crockery  is  a  veiy  frequent  means  of  introducing  these  seeds.  Thistles 
are  brought  to  the  barn  in  ha}'  or  grain.  The  seeds  reach  the  straw 
stack  f)r  manure  heap  and  are  taken  back  to  the  fields.  They  are  car- 
ried from  field  to  field  by  harvesting  machiner}'  and  from  farm  to  farm 
))y  thrashing  machines.  In  one  county  in  Oregon  the  Canada  thistle 
was  first  noticed  where  a  thrashing  machine  from  the  East  was  first 
usc(l.  It  would  have  cost  less  than  10  cents  to  have  cleaned  the  ma- 
<-hine  before  it  left  the  thistle-infested  region  where  it  had  been  used  in 
the  East,  but  it  would  now  probabh'  cost  thousands  of  dollai-s  to 
exterminate  the  thistles  that  have  sprung  from  that  introduction  of 
seeds. 

Disseniindfion  in  field  seeds. — As  an  impurity  in  commercial  seeds, 
Canada  thistle  seeds  are  found  most  frequently  in  Canada  blue  grass, 
Kentucky  })lue  grass  adulterated  with  Canada  blue  grass,  and  in  the 
clovers,  especially  alsikc.  If  the  thistles  are  cut  and  thrashed  with  any 
of  the  clovers  or  grass  seeds  it  is  often  im])ossible  to  completely  sei)arate 
the  seeds.  The  i)appus  or  down  nevei-  (flings  to  i^erfect  thistle  s(»eds 
after  passing  through  a  thrashing  niacliinc.  The  seeds  are  so  nearh'  of 
tlie  same  color  as  those  of  Canada  blue  grass  that  tli(y  are  detected 
with  difliculty,  even  by  trained  eyes;  but  the  yell<>\v  thistle  spines, 
which  are  more  readily  s<'en,  indicate  when  picsent  that  the  seeds  may 
be  i)i-esenl  also.  Canada  thistle  seeds  are  sometimes  found  in  wheat 
an«l  oats,  but  theii-  presence  in  these  grains  indicates  very  careless  clean- 
ing, as  tliev  aic  so  small  that  they  ma\-  l»e  easily  separated  l)y  pr(»per 
SCI'eeniliL'. 


Distribution  in  hay. — During  the  past  fifty  years  the  transportation 
of  hay  has  been  one  of  the  most  potent  agencies  in  the  dissemination  of 
Canada  thistle  seeds.  During  the  war  immense  quantities  of  haj^  were 
shipped  from  thistle-infested  regions  to  the  armies  in  the  field.  Since 
that  time  baled  hay  has  been  shipped  very  extensively  to  lumber  camps 
and  to  workmen  constructing  railwaj^s.  An  evidence  of  this  is  left  in 
the  patches  of  Canada  thistles  about  deserted  lumber  camps  and  along 
new  railway  lines.  Thistles  may  frequently  be  found  in  the  bales  of 
hay  received  for  city  consumption  and  an  abundant  growth  of  thistles 
is  often  seen  on  farms  where  manure  from  city  stables  is  used. 

METHODS    OF    ERADICATION. 

A  great  mam*  methods  for  killing  the  Canada  thistle  have  been  de- 
vised, and  many  have  been  described  in  various  publications,  3'et  there 
still  seems  to  be  need  of  a  more  widely  disseminated  knowledge  of  those 
which  have  been  tried  and  found  successful.  Mr.  Ambrose  Stevens  in 
an  excellent  essay  on  the  Canada  thistle,  published  in  the  Transactions 
of  the  New  York  Agricultural  Society  for  1846,  states  that  no  entirely 
new  methods  for  the  destruction  of  the  thistle  had  been  discovered  or 
developed  for  at  least  forty  years  previous  to  that  time.  He  gives  a 
summary  of  about  twenty-five  different  methods,  the  details  and  results 
of  which  had  been  published  in  agricultural  papers.  Each  of  these 
methods  had  proved  successful  and  each  had  proved  unsuccessful,  show- 
ing a  wide  variation  in  results  from  the  same  treatment  where  no 
account  was  taken  of  the  surrounding  conditions.  All  of  the  methods 
there  discussed  are  still  used  and  are  still  meeting  with  success  and 
failure.  Mr.  Stevens's  conclusions,  deduced  from  a  careful  studj-  of  his 
own  experiments  and  from  those  of  others,  are.  in  general,  applicable 
to  present  conditions  throughout  the  thistle-infested  region,  although 
experience  has  proved  some  of  the  statements  to  be  too  sweeping. 
They  are  as  follows : 

Whatever  will  effectually  exclude  the  plant  from  the  light  and  air  will  destroy 
it.  This  may  be  done  by  plowing,  in  some  soils,  and  in  others  by  a  close  grass 
sod.  ]*lowing,  if  repeated  frequently  in  soils  where  the  root  does  not  descend 
beyond  the  reacli  of  the  plowing,  will,  in  dry  seasons,  always  destroy  the  thistle, 
and  often  in  moist  ones.  In  soils  which  are  light,  deep,  rich,  friable,  and,  of 
course,  permeable  to  the  air,  and  are  in  some  measure  always  moist,  plowing 
will  always  fail. 

Wherever  a  dense  sod  can  be  formed,  Ihe  thistle  may  be  destroyed  by  seeding. 
The  grasses,  wherever  they  are  adapted  to  tiie  purpose,  will  be  found  the  easiest 
means  of  destruction,  although  not  so  rapid  as  ])lowing,  hoeing,  salting,  or  burn- 
ing, wliere  these  latter  are  available. 

In  all  ui)lands,  where  the  soil  is  of  a  (h'ptli  achnitting  the  root  to  be  reached 
and  affected  in  its  whole  extent  by  the  ]iIow,  hoe,  fire,  or  salt,  the  thistle  may 
be  destroyed  by  these  means,  and  they  will  be  found  the  most  rapid  ones. 

In  all  bottom  lands  where  the  root  descends  deep  and  the  soil  permits  access 
of  air,  neither  the  plow,  hoe,  (ire,  nor  salt  will  destroy  the  thistl(>:  licrc  the 
grasses  should  l)e  a]i])1ied,  and  will  be  found  the  best  destntyers. 


10 

Mowing  will  destroy  those  parts  of  the  thistle  wiiich  have  thrown  up  Howering 
stalks,  and  will  not  in  the  least  affect  those  which  have  not.  Mowing  should 
take  place  when  the  plant  Is  in  bloom. 

Whatever  limits  the  thorough  application  of  the  means  of  destruction,  will 
proportionately  diminish  success.  Hence  it  will  be  found  ditlicult  in  very  stony 
grounds  ever  to  eradicate  the  thistle  ;  the  plow  can  not  effectually  reach  its  roots, 
and  such  ground  is  rarely  a  good  grass  bearer.  Salt  and  sheep,  with  the  scythe, 
will  be  found  best  for  stony  grounds.  In  grounds  tilled  with  stumps,  where  the 
soil  is  rich  and  will  grow  a  dense  sod,  the  grasses  will  be  best,  and  in  such  the 
plow  should  not  be  used,  as  it  will  not  effectually  reach  all  the  roots.  Fences 
that  obstruct  the  application  of  the  plow  or  hoe  should  be  removed. 

If  it  be  desirable  to  destroy  the  thistle  by  the  grasses  it  will  be  found  best  to 
make  the  land  rich  by  manure.  This  will  force  the  grass  and  enable  it  more 
readily,  by  vigorous  growth,  to  kill  the  plant.  And  in  the  application  of  all 
remedies  care  should  be  taken  to  reduce  the  soil  by  proper  cultivation  to  a  fine 
tilth,  that  all  the  seeds  of  the  thistle  in  the  ground  may  germinate  and  not  lie 
dormant.  The  seed  is  very  hardy,  and  escapes  all  the  ordinary  means  of 
destruction,  except  fire. 

The  following  specific  methods  of  treatment  have  been  found  most 
successful  in  subduing  or  destroying  the  Canada  thistle : 

Mowing  twice  each  year,  just  after  the  flowers  open,  usually  in  June 
and  August,  will  keep  the  plants  in  subjection.  This  will  prevent  the 
production of  seeds,  and  thus  serious  injury  to  crops  may  be  avoided, 
}>ut  it  will  rareh'  cause  the  death  of  the  thistle  roots  except  in  good 
grass  land  or  in  wet  seasons,  and  will  therefore  need  to  be  repeated  each 
year.  It  is  generally  as  effective  as  pulling  or  grubbing  twice  a  year. 
This  plan  is  recommended  for  roadsides  and  waste  land,  and  for 
meadows  and  pastures  where  the  methods  'for  complete  eradication  seem 
to  be  too  expensive  for  innnediate  application.  C'anada  thistle  plants 
are  often  killed  b\'  mowing  them  just  as  a  heavy  rain  sets  in  late  in 
June  or  early  in  July,  when  they  are  in  bloom  and  the  stalks  are  hollow. 
The  rain,  keeping  the  cut  surface  moist  and  filling  up  the  hollow  stalks, 
favors  the  growth  of  fungi,  inducing  decay,  which  often  extends  down 
to  the  root  system. 

A  more  effective  method,  especiallx'  in  djy  seasons,  is  to  go  over  the 
ground  once  during  every  two  weeks  al'tci'  the  mowing  in  .hinc  and  cut 
off  every  thistle  about  two  inchc^s  Ix'low  the  surface  with  a  lioi'  or  spud. 
A  spud  made  of  a  strong,  sliarp  chisel  on  the  end  of  a  pitchfork  handle 
will  be  foimd  most  convenient  h)r  this  work.  The  second  year  the 
spudding  should  begin  as  soon  as  the  thistles  show  in  tlu*  spiing,  and 
should  be  continuecl  througli  tlui  season,  although  theic  will  l>e  few  to 
cut  after  niidsinnmer.  if  the  work  has  been  well  done.  'I'lie  land  should 
\)(t  looked  over  occasionallN'  each  yeai"  aflerwaids  to  detect  and  desti'oy 
plants  that  may  spring  from  dormant  se<'ds. 

Salting  thistle  plants  every  week  oi-  \\\n  during  two  successive  grow- 
ing seasons  in  pjistures  where  sheep  have  access  to  them,  usually 
destrovs  them. 


11 

Small  patches  of  the  plant  have  been  killed  by  covering  them  with 
straw,  tanbark,  or  apple  pomace ;  but  these  methods  can  not  be  rec- 
ommended. Canada  thistle  roots  will  live  for  three  years  or  longer  in 
porous  soils  under  straw  stacks  or  piles  of  tanbark,  and  the}'  are  likeh- 
to  creep  out  and  send  up  shoots.  Apple  pomace,  applied  thick  enough 
to  kill  the  thistle,  ruins  the  land  for  the  growth  of  any  crop  for  several 
3^ears ;  but  this  period  may  be  shortened  by  repeated  applications  of  air- 
slaked  lime. 

The  application  of  chemicals  or  some  substance  that,  being  absorbed 
into  the  tissues  of  the  plant,  will  kill  the  roots,  is  recommended  as  one 
of  the  best  methods  for  destro3dng  small  patches.  Trials  on  a  small 
scale,  proving  the  ease  and  effectiveness  of  this  method,  may  encourage 
its  extension  to  larger  areas.  The  following  substances  given  in  the 
approximate  order  of  their  effectiveness,  beginning  with  the  poorest, 
have  been  used  for  this  purpose  :  Salt,  brine,  quicklime,  kerosene, 
gasoline,  turpentine,  lye,  sodium  arsenite,  carbolic  acid,  muriatic  acid, 
nitric  acid,  sulphuric  acid.  Salt,  brine,  and  quicklime  are  most 
effective  when  applied  liberally  to  places  where  the  thistles  have  been 
grubbed  out.  Brine  is  often  applied  hot  with  good  effect.  Salt  and 
kerosene  are  often  used  together.  Kerosene,  gasoline,  turpentine,  and 
lye  may  be  applied  in  the  above  manner,  or  they  may  be  poured  into 
the  hollow  stems  when  the  plants  are  cut  in  flower.  This  process  is 
too  laborious  to  be  recommended,  except  in  case  of  small  patches. 
Strong  lye  and  the  other  substances  mentioned  will  be  found  effective 
if  applied  to  the  top  of  the  plants  when  thej^  are  growing  most  rapidh- 
during  May  and  June.  None  of  these  substances,  except  salt,  injure 
the  land  to  any  appreciable  extent  if  applied  only  in  sufBcient  quan- 
tities to  kill  the  thistles.  Salt  must  be  applied  in  such  large  quantities 
that  in  some  cases  it  may  not  be  washed  out  of  the  soil  for  two  or  three 
years.  The  stronger  acids  and  alkalies  are  somewhat  difficult  and 
dangerous  to  handle,  because  of  their  corrosive  properties.  Thej'  have 
to  be  stored  and  applied  in  glass  bottles.  Crude  sulphuric  acid,  which 
is  much  used  in  eastern  Pennsylvania,  is  applied  by  means  of  a  glass 
bottle  with  a  glass  tube  or  a  clay  pipestem  running  through  the  cork. 
Of  these  strong  chemicals,  a  few  drops  applied  to  each  plant  are  suffi- 
cient. Carbolic  acid  and  the  less  corrosive  substances  may  be  applied 
b}^  means  of  an  ordinary  machine  oil  can,  or  a  watering  pot  with  a 
small  rose  or  nozzle.  A  teaspoonful  of  strong  commercial  carbolic 
acid  applied  to  each  plant  is  sufficient.  This  should  ])e  applied  without 
dilution  on  the  buds  and  tender  upper  leaves  of  the  growing  plant. 
Care  should  be  exercised  to  prevent  the  poisoning  of  stock  from  chem- 
icals applied  to  thistles  in  pasture  fields. 

Patches  of  Canada  thistles  discovered  in  grain  fields  at  harvest  time, 
as  they  often  are,  should  be  left  standing  until  after  the  cro])  is 
removed,  then   mowcnl  and   l)urned    on    the   spot   as   soon   as  they  are 


12 

dry  enough.  This  treatment  arrests  the  distribution  of  seeds  and,  in 
some  cases,  it  has  killed  the  plants. 

In  shallow,  dr}-  soils  summer  fallowing  during  a  dry  season  will 
destroy-  the  thistles. 

The  first  plowing  should  be  done  when  the  plants  are  in  bloom  in 
June  or  earlj'  in  July.  If  they  can  not  be  turned  under  cleanlj-  with 
chain  or  jointer,  they  should  be  mowed  and  burned  before  plowing. 
The  land  should  be  alternately  harrowed  and  cross-plowed  as  often  as 
any  green  plants  appear  until  it  is  time  to  sow  winter  grain.  Thorough 
cultivation  with  hoed  crops  will  produce  almost  the  same  effect  if  the 
cultivation  is  continued  through  the  summer.  In  this  case  a  hoe  must 
be  used  to  destroy-  thistles  growing  in  the  hills  and  others  that  escape 
the  cultivator.  In  wet  seasons  cultivation  generalh'  fails  to  kill  the 
roots.  After  cultivating  either  in  barren  fallow  or  with  hoed  crops,  the 
land  should  be  thickly  seeded  in  August  or  September  with  crimson 
clover,  rj'e,  or  winter  oats  where  the  winter  climate  will  permit  the 
growth  of  these  crops.  These  may  be  pastured  during  the  early  spring 
and  then  plowed  under.  Winter  wheat  and  other  grain  crops  that  will 
permit  the  thistle  to  remain  undisturbed  during  spring  and  earh'  sum- 
mer should  not  be  grown.  Where  the  <jlimate  is  too  severe  for  winter 
crops,  cultivation  should  continue  late  in  the  fall  and  begin  early  in 
spring.  Thistle-infested  ar^as  ought  to  be  plowed  and  cultivated  b}- 
themselves  to  avoid  scattering  roots  to  other  parts  of  the  field. 

Canade  thistles  that  persist  in  spite  of  cultivation  on  low  lands  may 
soon  disappear  when  the  land  is  seeded  and  made  to  produce  two  good 
crops  of  hay  each  year.  On  soils  not  adapted  for  permanent  grass 
lands  it  is  often  possible  to  raise  good  crops  of  annual  grasses  which 
will  choke  out  the  thistles.  Millet,  fodder  corn,  or  sorghum  are  good 
crops  for  this  purpose,  and  good  results  have  l)een  obtained  by  the  cul- 
tivation of  rape. 

Dr.  T.  J.  P>urrill,  in  Bulletin  No.  12,  Illinois  Agricultural  Experi- 
ment Station,  recommends  the  following  method  as  the  "best  for 
exterminating  Canada  thistles  when  in  full  possession  of  tillable  ground: 

"1.  Cut  the  thistles  wlicii  in  full  bloom  [July]  as  close  to  the  ground 
as  possible. 

"2.  Plow  about  'i  inches  deep  and  sow  millet  or  Hungarian  grass, 
seeding  heavily;  harrow.  'I'liis  may  follow  (he  i)receding  at  once  or 
after  some  two  w(M;ks'  delay. 

"  .S.  In  Sei)teml)er  plow  under  the  ciop  or  save  it  for  hay.  as  desired. 
At  all  events,  plow  and  seed  libei'ally  with  rye. 

"  4.  Plow  under  the  rye  in  May  and  seed  again  with  millet  or  Hungarian 
grass,  or  i)lant  to  some  hoed  crop  [cornl  and  give  the  most  thorough 
cultivation,  with  continual  seaicliing  for  and  destruction  of  every 
remaining  thistle. 


13 

"5.  Continue  the  clean  cultivation  and  sharp  lookout  for  thistles 
another  j^ear." 

NATURAL   ENEMIES. 

Although  the  larvae  of  several  different  kinds  of  insects  live  in  the 
stems  of  Canada  thistles,  while  others  feed  more  or  less  upon  the  roots 
and  still  others  eat  the  foliage,  they  seem  to  produce  comparatively 
little  effect  upon  the  vigor  or  productiveness  of  the  plants.  The 
American  goldfinch  or  "yellow  bird,"  often  called  the  "thistle  bird," 
is  sometimes  unjustly  accused  of  scattering  thistle  seeds.  It  does 
scatter  the  down  from  which  it  has  detached  the  seeds.  Thistle  seeds 
form  one  of  its  favorite  kinds  of  food,  and  it  is  undoubtedly  one  of  the 
best  natural  agencies  tending  to  keep  the  thistle  in  check. 

Among  fungus  diseases  which  attack  Canada  thistle,  the  thistle  rust, 
Puccinia  suavfolens,  is  the  most  destructive.  This  often  prevents  the 
production  of  seeds  and  sometimes  kills  the  plant  to  the  ground.  It 
is  most  effective  during  wet  seasons,  but  even  under  the  most  favorable 
conditions  it  rarely  spreads  so  as  to  destroy  all  of  the  plants  in  a  patch. 
Experiments  made  thus  far  in  New  York,  New  Jersey,  and  Pennsyl- 
vania, in  introducing  the  rust  into  uninfested  patches,  indicate  that 
while  it  may  often  aid  materially  in  checking  the  growth  of  the  thistles, 
it  rareh^  exterminates  them,  and  its  action  is  too  uncertain  to  warrant 
more  than  a  qualified  recommendation  of  its  use  as  a  thistle-destroj^ng 
agent. 

STATE    LAWS    RELATING    TO    CANADA    THISTLE. 

Canada  thistle  is  proscribed  as  a  noxious  weed  by  the  laws  of  the 
following  twentj-four  States : 

California.  Kansas.  Nebraska.  Pennsylvania. 

Connecticut.  Kentucky.  New  Jersey.  South  Dakota. 

Delaware.  Maryland.  New  York.  Vermont . 

Illinois.  Michigan.  North  Dakota.  Washington. 

Indiana.  Minnesota.  Ohio.  Wisconsin. 

Iowa.  Missouri.  Oregon.  Wyoming. 

In  most  of  these  States  penalties  are  prescribed  for  permitting  the 
thistle  to  produce  seeds.  Illinois  is  the  only  State  in  which  the  law 
directs  that  the  plants  be  killed,  and  this  also  the  only  State  in  which 
the  law  appears  to  be  vigorously  enforced.  In  several  counties  in 
different  parts  of  the  State  Canada  thistles  are  reported  as  practically 
exterminated  through  the  rigid  enforcement  of  the  law.  In  some  of 
the  large  cities  of  this  country  where  there  are  thousands  of  acres  of 
vacant  lots  grown  up  to  thistles  and  other  weeds,  neither  the  thistles 
nor  the  thistle  laws  have  received  much  attention. 

The  majorit}'  of  progressive  farmers  know  that  Canada  thistle  can 
be  exterminated  on  their  farms,  but  they  need  the  aid  of  a  good  law. 
well  administered,  to  prevent  their  well-tilled  fields  from  being  seeded 


14 

b}'  the  thistle  patches  of  careless  neighbors.  While  it  is  not  regarded 
as  necessary-  or  desirable  that  the  Canada  thistle  should  be  treated  in 
a  law  distinct  from  laws  relating  to  other  weeds,  it  is  earnestl}-  recom- 
mended that  it  be  proscribed  b}'  just  and  comprehensive  laws  in  all  of 
the  States  where  it  is  at  present  abundant,  and  especialh'  in  those 
States  of  the  Rocky  Mountain  region  and  Great  Basin  where  it  is  now 
becoming  established.  An  effort  should  be  made  to  enforce  the  laws 
that  now  exist  unheeded  in  some  of  the  older  States,  and  if  they  are 
found  inadequate,  they  should  be  repealed  and  replaced  bj'  better  ones. 

Lyster  H.  Dewf.y, 

Assistant  Botanist. 
Approved  : 

James  Wilson, 

Secretary  of  Agriculture. 

Washington,  D.  C,  June  17,  1901. 

O 


n 


UNIVERSITY  OF  FLORIDA 


3  1262  09221  4864 


